Question

Найдите наименьшее значение функции y= (x^3-128)/x на отрезке [-8;-2]

Answer 1

$y=\frac{x^3-128}{x}\; ,\; \; ODZ:\; \; x\ne 0\\\\x\in [\, -8,-2\, ]\\\\y=x^2-\frac{128}{x}\\\\y'=2x+\frac{128}{x^2}=\frac{2x^3+128}{x^2}=\frac{2(x^3+64)}{x^2}=\frac{2(x+4)(x^2-4x+16)}{x^2}=0\\\\(x+4)(x^2-4x+16)=0\; ,\; \; x\ne 0\\\\x=-4\; \; \; (x^2-4x+16\ \textgreater \ 0\; ,\; t,k,\; D=16-64\ \textless \ 0)\\\\---(-4)+++(0)+++\\\\.\; \; \searrow \; \; \; (-4)\; \; \nearrow \; \; \; \; (0)\; \; \; \nearrow \\\\x_{min}=-4\\\\y(-8)=80\\\\y(-4)=48\\\\y(-2)=68\\\\y_{naimenshee}=y(-4)=48$